Probably one of the earliest sources is the Midrash Osios d'Rabbi Akiva, Os Beis (eighth or ninth-century) which reads:
דבר אחר מפני מה ברא הקב"ה את העולם בב"י בראשית ומסיים לעיני כל ישראל,
שכשתדבקם יחד אינן אלא ב"ל וכשהפכת אותם אינו אלא ל"ב, אמר הקב"ה לישראל,
בני אם אתם מקיימים שתי מדות הללו בל ולב מעלה אני עליכם כאלו קיימתם כל
התורה כולה מב"י ועל למ"ד.
Why did HKBH create the world with ב and end with ל?
When you put them together they result בל (not) and when you
reverse them around they read לב (heart). Thus, HKBH says to
Yisroel: If you serve me in these two ways, with your nothingness (humility)
and with your heart (kavod) I will consider it as if you fulfilled
the entire Torah from ב to the ל.
On the reverse reading of לב , there is an interesting piece in Letters of Fire by Matityahu Glazerson (NY: Feldheim, 1991, p. 15). He says:
When read in reverse order, the word לב becomes בל (bal, “not”),
indicating negation. One's life depends upon the heart; if one uses
his heart properly, it can bring him to perfection. Otherwise, God
forbid, it can bring him a feeling of lack and negation. Kohelet says:
“The heart of the wise man is to the right, and the heart of the fool
is to the left." That is, when we read the word from the right, לב
(lev, "heart"), it shows the way of the wise; from the left, בל (bal,
"not"), it shows the way of the fool.