Different categories of hadith (sayings attributed to the Islamic prophet Muhammad) have been used by various scholars. The muhaddithun (experts in the science of hadith criticism) generally use two terms - taqrīr for tacit approvals, and khabar for sayings and acts ascribed to Muhammad.The term taqrīr implies that, in the presence of Muhammad, a believer did something, which Muhammad noticed but did not disapprove or condemn.