Expressing happiness and celebrating the beloved Prophet śall Allāhu ‘alayhi wa sallam his birthday causes even unbelievers, by Allah’s favour and mercy, to gain some benefit. This is mentioned in Sahīh Bukhârī. Bukhārī said in his Hadīth that every Monday, Abū Lahab in his grave is released from punishment because he freed his handmaid Thuwayba when she brought him the news of the Prophet’s śall Allāhu ‘alayhi wa sallam birth.

This Hadīth is mentioned in Bukhārī in the book of Nikah, and Ibn Kathīr mentions it in his books Sirāt al-Nabī (1:124), Mawlid al-Nabī (pp. 21), and al-Bidāya (pp. 272-273). The Hāfidh Shams ud-Dīn Muhammad ibn Nasir ud-Dīn ad-Dimashqī wrote on this the following verses in his book Mawrid as-Sadi fi Mawlid al-Hadi:

If this, a disbeliever who was condemned to hell eternally with ‘Perish his hands’ [Sūrah 111], is said to enjoy a respite every Monday because he rejoiced in Ahmad śall Allāhu ‘alayhi wa sallam: what then do you think of the servant who, all his life, was happy with Ahmad, and died saying, ‘One’?