Quoting “The Letter of Imaam Ahmad to Musaddad ibn Musarhad”, which was narrated by Qaadhi Abu Ya’la Muhammad ibn Muhammad ibn al-Husayn ibn Khalf al-Farraa’ narrates in at-Tabaqaat [At-Tabaqaat : 1/342.] and Abu al-Faraj Abd ar-Rahmaan ibn Ali ibn al-Jawzi al-Hanbali in his book Manaaqib al-Imaam Ahmad [Al-Manaaqib p. 217.] and al-Qaadhi Burhan ad-Deen Ibraaheem ibn Muflih mentioned in his book Al-Maqsad al-Arshad from Abu Bakr Ahmad ibn Muhammad al-Burda’i at-Tameemi .
Some have challenged the authenticity of attributing this treatise to Imaam Ahmad on the grounds that Ahmad ibn Muhammad al-Burda’i at-Tameemi is unknown. However Shaykh al-Islaam ibn Taymiyyah has lifted any doubt surrounding its reliability:
“As for the Letter of Ahmad ibn Hanbal to Musaddad ibn Musarhad, it is well known to the People of Hadeeth and Sunnah from amongst the companions of Ahmad and other than them, they took it with acceptance. And Abu Abdullaah ibn Battah mentioned it in his book al-Ibaanah and more than one has relied upon it such as al-Qaadhi Abu Ya’laa and he wrote it by his own hand.” Majmoo’ al-Fatawaa , vol. 5, p. 396. For the full discussion surrounding this letter, see Majmoo’ al-Fataawaa, vol. 5, pp. 375-377, 380-396.
So in this letter imam Ahmad said:
And as for the Raafidhah : then those whom we have met from the people of knowledge are in agreement that they say: Ali ibn Abu Taalib is more virtuous than Abu Bakr as-Siddeeq, and that the (conversion to) Islaam of Ali was older than the (conversion to) Islaam of Abu Bakr, so whoever claims that Ali ibn Abu Taalib is more virtuous than Abu Bakr, then he has rejected the Book and the Sunnah. Allaah, the Most High, states: “Muhammad is the Messenger of Allaah and those who are with him…” al-ayah 
So Allaah gave precedence to Abu Bakr after the Prophet, not Ali, and the Prophet, sallallaahu alayhi wa sallam, said: “If I was going to take a khaleel , I would have taken Abu Bakr as a khaleel , however, Allaah has taken your companion as a khaleel – that is, himself- and there is no prophet after me.” 
And whoever claims that the Islaam of Ali was before the Islaam of Abu Bakr, then he is mistaken for Abu Bakr converted and he was at that time thirty-five years of age, while Ali was at that time seven years of age, and the commands and punishments and obligations did not yet apply to him.
1) The rest of the verse states, “… are severe with the disbelievers, merciful amongst themselves. You see them bowing and prostrating, seeking bounty from Allaah and His Pleasure. Their mark (of their faith) is in their Faces from the traces of prostration. This is their description in the Torah, and their description in the Gospel is like a (sown) seed which sends forth its shoot, then makes it strong, and becomes thick and it stands straight on its stem, delighting the sowers, that he may enrage the disbelievers with them. Allaah has promised those among them who believe and do righteous deeds forgiveness and a great reward.” [al-Fath (48):29] It was a common practice in the works of classical scholars to simply quote part of a verse and then simply state “al-ayah” to indicate the rest of the ayah as it was understood that the reading audience would generally be acquainted with the rest of the verse.
2) Muslim reported it (no. 2383) – in Virtues of the Companions – from the hadeeth of ibn Mas’ood with the wording, “If I was going to take a khaleel , I would have taken Abu Bakr as a khaleel , but he is my brother and my companion, and Allaah, Azza wa Jall, has taken your companion as a khaleel .” And concerning the topic there is a narration from Jundub ibn Abdullaah (no. 532) and from Ibn Abbaas with Bukhaari (7/15) and from Abu Sa’eed al-Khudri with Bukhaari (7/10-11) and Muslim (2382) and at-Tirmidhi (3661).