"النِّکاحُ سُنَّتِي، فَمَن رَغِبَ عَن سُنَّتي فَلَیسَ مِنّي"
"Marriage is my sunnah (tradition). Whosoever does not follow my sunnah is not from me."
The Prophet (pbuh) is quoted to have said,
Marriage is my sunnah (tradition). Whosoever does not follow my sunnah is not from me.1
Sunnah is an Arabic word which means a path or a way. Sunnah is one of the sources of Islamic law and includes the sayings, actions and tacit approvals of Prophet Muhammad (pbuh). Whoever wants to be a true Muslim must follow the sunnah of the Prophet (pbuh).
The hadith mentioned above indicates that marriage is not just a social practice but it complements one's religion, that is why starting a family is so much encouraged in Islam.
Source:
1. Muhammad ibn Muhammad Shu'airi, Jami' al-Akhbar, Najaf: Haidariyyah Publications, p. 101.