Ibn al-Qayyim said in Ialaam al-Muwaqqiin:

Imam Ahmad said in the narration of his son, Salih: “When a man takes upon himself the responsibility of issuing fatwa, it is necessary for him to know the explanations (wujuh) of the Qur?an, know the authentic (sahih) chains of transmission and know the traditions (sunan).” He said in the narration of Abu al-Harith: “Fatwa is not permissible except for a man well-versed in the Book and Sunnah.”

He said in the narration of Hanbal: “The one who issues fatwa must know the opinions of those who came before. Otherwise he may not issue fatwa.” He said in the narration of Yusuf ibn Musa: “It is necessary on the man [who issues fatwa] to learn all that the people [of knowledge] differed on.”

He said in the narration of Muhammad ibn „Ubayd Allah ibn al-Munadi, when he heard a man asking him, “If a man memorises a hundred thousand hadiths, does he become a jurist (faqih)?” He said, “No.” He said, “Then, two hundred thousand?” He said, “No.” He said, “Then, three hundred thousand?” He said “No.” He said, “Then, four hundred thousand?” He gestured with his hand “like so,” moving it.

„Abd Allah ibn Ahmad said: “I asked my father about a man who possesses books compiled on the sayings of Allah?s Messenger (Allah bless him and grant him peace) and the Sahabah and Tabi„in but the man has no insight into the weak and abandoned hadith, nor the strong isnad (chain of narration) from the weak isnad, is it permissible for him to act upon what he pleases and choose therefrom, issuing fatwa upon it and acting upon it? He said: He cannot act upon it until he asks about what is admissible from them, in order that he acts upon an authentic command, having sought that from the people of knowledge.?”

These are unequivocal statements from Ahmad, who was a mujtahid Imam and was the most pursuant of hadith from the people [of knowledge] and the one most acquainted with it according to Ibn al- Qayyim, directing you to the obligation of taqlid for the layperson of an „alim who combines all the conditions of ijtihad, consisting of knowledge of the Book of Allah, the traditions of His Messenger (Allah bless him and grant him peace) and familiarity with the statements of the Salaf and the memorisation of four hundred thousand hadiths and other than that of which no mention is made here.

Source: Al-Din Al-Qayyim-Defence On Taqlid