Aboo Ya'la has told us that he said that Aboo Musa Muhammad ibn Muthana has said that Wahab ibn Jurair said that his father said I have heard Muhammad ibn Ishaq talking from Salih ibn Kiyas from Ubaid Allah ibn Abdullah that he said, ''I saw Usamah ibn Zayd praying at the grave of the Prophet (saw). Marwan ibn Al-Hakam entered and said: "Do you pray to a grave?" So he (Zayd) said: "I love him!" So (Marwan) said some repulsive words. So he turned and Usama walked and said: "O Marwan! You have hurt me, and I have heard Prophet (saw) saying, ''Allah (swt) hates the vile, the doer of obscene act and you are vile and a doer of obscene act."
Source: Saheeh ibn Hibban. Vol. 12, Pg. # 506, H. # 5694.
Is the Hadith above authentic? It contradicts the Prophetic prohibition of praying at graves. Could it be that Usama ibn Zayd RA was unaware of said prohibition?