Musnad Ahmad 1689

It was narrated from "Ubaidullah bin ‘Abdullah, from Ibn "Abbas, that he was discussing the issue of prayer with ‘Umar. "Abdur-Rahman bin ‘Awf came to them and said: Shall I not tell you of a hadeeth that I heard from the Messenger of Allah (ﷺ)? They said: Yes. He said: I bear witness that sheard the Messenger of Allah (ﷺ) say: “Whoever offers a prayer and thinks that he did less than he should, let him continue praying until he thinks that he did more than he should."