(30) باب مَنْ أَجَابَ بِلَبَّيْكَ وَسَعْدَيْكَ
Narrated Mu`adh:
While I was a companion rider with the Prophet (ﷺ) he said, "O Mu`adh!" I replied, "Labbaik wa Sa`daik." He repeated this call three times and then said, "Do you know what Allah's Right on His slaves is?" I replied, "No." He said, Allah's Right on His slaves is that they should worship Him (Alone) and should not join partners in worship with Him." He said, "O Mu`adh!" I replied, "Labbaik wa Sa`daik." He said, "Do you know what the right of (Allah's) salves on Allah is, if they do that (worship Him Alone and join none in His worship)? It is that He will not punish them."(another chain through Mu'adh)
Reference: Sahih al-Bukhari 6267
In-book reference: Book 79, Hadith 41
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 74, Hadith 283