Question:  Should one say Rabbana LAKAL Hamd or WALAKAL Hamd?

Answer: (1) The author of Al-Muhith said:

According to Imam Shafie r.a it is said with waw. And if it is said without waw is still is sufficient. And Abu Hafs r.a said: there are no distinction in saying لَكَ and ولَكَ.

(2) On the contrary Imam Abu Hanifa r.a says it should be said without ‘waw’

(Rabbana lakal hamd) as it is stated from the hadith of Abu Awfa r.a. which has been narrated by Muslim and others.

References: [1]Bukhari: 796 [2] Tirmizi:268 [3] Ahmad:7190 [4] Abu Dawud:848 [5] Sunan Nasai:1063 [6] Muatta Malik:44 [7] Nukhab, vol-4, p-285