Sahih Bukhari Volume 002, Book 026, Hadith Number 821.
Narated By Khalid bin Al-Harith : 'Ubaidullah was asked about Al Mahassab. 'Ubaidullah narrated: Nafi' said, 'Allah's Apostles, 'Umar and Ibn 'Umar camped there." Nafi' added, "Ibn 'Umar used to offer the Zuhr and 'Asr prayers at it (i.e. Al-Mahassab)." I think he mentioned the Maghrib prayer also. I said, "I don't doubt about 'Isha' (i.e. he used to offer it there also), and he used to sleep there for a while. He used to say, 'The Prophet used to do the same.'"