Parashurama and Bhishma, two of the greatest warriors of the epics, shared an interesting history together as guru and student. Bhishma was a Kuru prince, and Parashurama instructed him in the martial arts as a boy. Their conflict began years later, with the beautiful princess Amba. Along with her sisters Ambika and Ambalika, princess Amba had been abducted by Bhishma in a misunderstanding between two kingdoms. Now, with her honor tainted, no man would take her for bride, and she was condemned to remain destitute. Bhishma himself was unable to marry due to his bhishana pratigya, or vow of celibacy, and allegiance to Hastinapur. Amba then sought the help of Parashurama to kill Bhishma.
Taking pity on her plight, the avatar agreed to fight his former student on her behalf. The battle lasted twenty-three days, by the end of which, both warriors were bloodied and filled with arrows. Bhishma had knowledge of the divine deadly weapon Prashwapa, which had the power to put a foe to sleep, and of which Parashurama was unaware. When he was about to use the celestial weaponry, all Gods rushed to Bhishma and asked him to hold his hand, as it would humiliate his guru. Out of respect, Bhishma acquiesced.
Pitrs then appeared and obstructed the chariot of Parashurama, forbidding him from fighting any longer. The spirit of Parashurama’s father, Jamadagni and his grandfather, Rucheeka, spoke to him:
O son, never again engage in battle with Bhishma or any other Kshatriya. Heroism and courage in battle are the qualities of a Kshatriya, and study of the Vedas and the practice of austerities are the wealth of the Brahmans. Previously you took up weapons to protect the Brahmans, but this is not the case now. Let this battle with Bhishma be your last. O son of the Bhrigu race, it is not possible to defeat Bhishma.
—Mahabharata 188:5
In the end, the Gods showered praise on Bhishma, and he sought the blessing of Parashurama as his guru. The avatar then acknowledged that his former student was truly invincible, telling Amba:
Using even the very best of weapons I have not been able to obtain any advantage over Bhishma, that foremost of all wielders of weapons! I have exerted now to the best of my power and might. Seek the protection of Bhishma himself, thou hast no other refuge now.
—Mahabharata 189:1
Source – Wikipedia
Karna The Great 