It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When any one of you enters the mosque, he should not sit down without offering two rak‘ahs." Imam Abu Bakr (may Allah have mercy on him) says that this is a lengthy chapter. I have explained it in Kitab al-Kabir. Imam Abu Bakr (may Allah have mercy on him) says that this command is for virtue and reward, not for obligation and compulsion. The evidence for this is the hadith of Sayyiduna Talhah bin Abdullah (may Allah be pleased with him), which he narrates from the Noble Prophet (peace and blessings be upon him): When he (peace and blessings be upon him) mentioned the obligation of the five prayers, a man asked, "Is there any prayer obligatory upon me besides these?" He (peace and blessings be upon him) replied, "No, except that you offer voluntary prayers." In this way, he (peace and blessings be upon him) clarified that besides the five daily prayers, there are voluntary prayers, not obligatory ones.