Abdullah bin Masud reported,

We used to greet the Prophet ﷺ with the salam when we were in Mecca, before we journeyed to the land of Abyssinia. When we returned from Abyssinia, we came to him and greeted him, but he did not return our greeting. I was overcome with worry, fearing every possible reason, until they finished the prayer. Then I asked him about it, and he said, "Allah brings about whatever He wills in His command, and He has now decreed that we are not to speak during the prayer."

حدثنا سفيان عن عاصم عن أبي وائل

عن عبد الله قال

كنا نسلم على النبي ﷺ إذا كنا بمكة قبل أن نأتي أرض الحبشة فلما قدمنا من أرض الحبشة أتيناه فسلمنا عليه فلم يرد فأخذني ما قرب وما بعد حتى قضوا الصلاة فسألته فقال إن الله عز وجل يحدث في أمره ما يشاء وإنه قد أحدث من أمره أن لا نتكلم في الصلاة