This title was given in history to some people known for their long prayers and sujud. Three people whom I know were sons of Sahaba referred to as al-Sajjaad due to their excess in Sujud.
1. Muhammad b. Talha b. ‘Ubaidullah: He died in the battle of Jamal while fighting from the side of his father. Ibn Sa’d and Ibn Abdul Barr mention that when ‘Ali b. Abi Talib (ra) found the body of Muhammad b. Talha in the battle field he said, “This is al-Sajjaad, by the Lord of ka’bah. His kindness to his father has killed him.” Imam Bukhari mentioned in al-Tarikh al-Kabeer that the Messenger of Allah (sallallahu ‘alaihi wa sallam) named him as Muhammad.Imam al-Dhahabi said: He was known as “al-Sajjad” due to his excess in Salaat and Ibadah.
2. ‘Ali b. Abdullah b. ‘Abbas al-Hashimi: The son of Ibn ‘Abbas was also known as al-Sajjad due to his worship and excess in sujud. He was the grandfather of al-Saffah the first Abbasid caliph. al-Dhahabi said: He was al-Imam, al-Sayyid, father of Khulafa, Abu Muhammad al-hashimi, al-Sajjaad. He was born in the year of the martrydom of al-Imam ‘Ali (b. Abi Talib), so he named on his name.” it is said that he would pray one thousand rak’ah of Salaah every day. He died in 118 AH.
3. ‘Ali b. Husain b. ‘Ali: He is well known as Zain al-‘Aabideen. He is also referred to as al-Sajjad among people but his title Zain al-‘Abideen was more famous. He is most famous and most loved among all the three. I have compiled a biography of him. He died in the year 94 AH.