It is narrated from Sayyiduna Muhajir bin Qunfudh (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) was urinating - or the narrator said: I passed by him when he had urinated - so I greeted him with salam, but he did not return my greeting until he had finished his ablution (wudu); then he returned my greeting.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17910