Masruq narrated: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Be still in the prayer." Mansur was asked: What is meant by that? He said: "That he should be fully settled in his prayer when he bows and when he prostrates."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9311