All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
First: The Hadeeth in question was narrated by Al-Bukhari and Muslim.
Second: There is no doubt that Sa‘d ibn Abi Waqqaas is one of the companions who were given the glad tidings of entering Paradise. Sa‘d might have said that because he had not heard that statement himself from the Prophet, sallallaahu ‘alayhi wa sallam; however, he did not deny that other companions may have heard it from the Prophet, sallallaahu ‘alayhi wa sallam.
Alternatively, he may have said so out of humbleness and to avoid bragging, just as ‘Abdullaah ibn Salaam did after being told that he is one of those given glad tidings of entering Paradise; he said, “One should not say what he does not know.” [Al-Bukhari and Muslim]