Source: Fatawa Aman Puri by Sheikh Ghulam Mustafa Zahir Aman Puri
Question:
What is the ruling on a walima before marriage?
Answer:
It is against Sharia. The walima is a Sunnah after the marriage. If someone performs a walima before the marriage, the blessings of the marriage may be nullified.
❀ The Prophet Muhammad (peace be upon him) said:
جعل الذلة والصغار على من خالف أمري
"Whoever disobeys my command, humiliation and disgrace will be imposed upon him."
Reference: (Musnad Ahmad: 50/2, its chain is good)
❀ Ahmad Raza Khan Barelvi says:
"The walima is a Sunnah of the 'night of the wedding' and even a minor should perform the walima of the wedding and it should be done on the morning after the wedding night."
Reference: (Malfoozat, Part Three, p. 262)