I have heard from an unauthoritative source that one should not interlace his fingers while praying, because, l'havdil, Christians do. Is there any authoritative source for the claim that it's inappropriate to interlace one's fingers while praying (or in general)? If so, what reason is given? (I know Orach Chayim 91 discusses the ideal hand configuration for praying the amida, and am not asking about that. I'm asking specifically about a disapproval of interlaced fingers, beyond any general disapproval of less-than-ideal hand configurations.)
A non Torah-observant artist once wanted to give the [Lubavitch] Rebbe a portrait he had painted of him. However, the Rebbe noticed that the picture showed him with his fingers intertwined, and he explained to the artist in terms that he would relate to as to why we do not interlace our fingers.
"Others may make this mistake and may even say that they saw in the picture that this is something that the Lubavitcher Rebbe does," the Rebbe added. The Rebbe asked the artist to change this detail, stating that he would only accept the portrait once this was done. The artist was only too happy to fulfill the Rebbe's request, but he asked the Rebbe his opinion on the portrait in general. "It's much better than the original," was the Rebbe's reply.
Edit: I think the prortrait is the first one here: http://raphaelnouril.com/portraits.aspx
The Aruch HaShulchan brings L'Halacha (OC 91:7) that in times of Za'am or raging tzorus, one should daven with his hands clasped together and his fingers interlocking. However says the Aruch HaShulchan in times of peace you should not do so because it causes Din Shamayim to be brought down on you.
The Piskei Tshuvos (95:5) brings that Arizal was makpid that you should never hold your hands in that manner and it is bad Mazal. Rav Chaim Palagi writes that one should stop his wife or children from doing this, ever, and especially in times of sickness or the Aseres Yimei Tshuva.
The Aruch HaShulchan says that although placing one hand inside the other is the best way to hold your hands by davening, each person is different and should keep his hands the way he feels comfortable, like on a Shtender or holding on to a shelf. However he says, make sure not to daven wearing gloves.
The Zohar Ha’kadosh, in Vayikra (p. 24), writes that when a harsh judgment is issued against a person, Heaven forbid, his fingers will unwittingly begin moving, and the fingers of his two hands will become interlocked. As interlocking hands is a sign of harsh judgment, it is improper for a person to intentionally hold his hands in this position. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Pinhas (18), cites this Halacha in the name of the Arizal (Rav Yishak Luria of Safed, 1534-1572). This is mentioned in other sources, as well, including the Sefer Ha’hasidim (by Rabbenu Yehuda Ha’hasid, Germany, d. 1217) and the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939). Thus, while it is permissible to place one hand on top of the other, one should not interlock the fingers of the two hands.
The Ben Ish Hai goes even further, writing that one should never place his hands behind his back, and should rather keep them in front of him at all times. Indeed, there are different kinds of spiritual powers associated with the positioning of one’s hands and fingers, as Rabbenu Bahya discusses at length.
The Sefer Hazechira mentions that those who are accustomed to interlocking their fingers run the risk of experiencing extreme anxiety, Heaven forbid.
There are many warnings of this kind that we hear as children, and many people are unable to distinguish between those that stem from folklore and superstition, and those which have a clear basis in Jewish tradition. When it comes to interlocking fingers, this is a well-documented warning that originates already from the Zohar and the teachings of the Arizal.
Summary: According to Kabbalistic tradition, one should not merge his hands together such that the fingers of the two hands interlock. According to some sources, one should also refrain from placing his hands behind his back.