Muwatta Malik: Chapter 4

Yahya related to me from Malik from Ibn Shihab from Abu Salama
ibn Abdar-Rahman ibn Awf from Abu Hurayra that the Messenger of Allah,
may Allah bless him and grant him peace, said, "When you stand in
prayer, Shaytan comes to you and confuses you until you do not know
how much you have prayed. If you find that happening do two sajdas
from the sitting position."

Yahya related to me from Malik that he had heard that the
Messenger of Allah, may Allah bless him and grant him peace, said, "I
forget or I am made to forget so that I may establish the sunna."

Yahya related to me from Malik that he had heard that a man
questioned al-Qasim ibn Muhammad saying, "My imagination works in the
prayer, and it happens to me a lot." Al-Qasim ibn Muhammad said, "Go
on with your prayer, for it will not go away from you until you go away
saying, 'I have not completed my prayer.' "