Narrated `Aisha:
Whoever tells that Muhammad concealed part of what was revealed to him, is a liar, for Allah says:-- "O Apostle (Muhammad)! Proclaim (the Message) which has been sent down to you from your Lord." (5.67) (PT)
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 136 (PT)
Arabic reference (PT): Book 65, Hadith 4612 (PT)