In general we say that to be "somech geulah l'tefilla" (connect redemption to prayer) it is required to start the Amidah immediately after finishing the bracha of Ga'al Yisrael in Shacharit, with no responses allowed between them.
I believe I heard at some point that some say the halacha on Shabbos is different, such that if you have not yet answered kedusha or barchu you should in fact make a hefsek (break) and answer.
Is this true? If so, what is the source?
