Mukhtasar Bukhari (Abridged) 2169 — Book of Blood Money

Narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them) that the Prophet (peace and blessings of Allah be upon him) said to (Sayyiduna) Miqdad (may Allah be pleased with him): "When a believer hides his faith among the disbelievers and then he reveals his faith and you kill him, then (this is not correct, this is indeed a great sin, and O Miqdad!) in Makkah, you too used to hide your faith in the same way."