Sufis (like Rawafid) post numerous sources for this fairy-tale. It is based on flimsy reports, filled with majahil and other defects. They will quote some books by some scholars (Hanbalis) like Ibn al-Qayyim and other sources, none of them are authentic.
Imam Dhahabi rejected and weakened this nonsense, but the Sufis (for obvious reasons) peddle these khurafat (superstitions) as they need to justify the ghuluw with their own pirs/shaykhs.
The khurafi Sufi Ibn ‘Alawi quotes this flimsy report in one of his books. Keep in mind, he is one of the top Sufi hadith scholars of our age, a Maliki in fiqh, yet he is ignorant of what the scholars said about this nonsense.
Imam Dhahabi said regarding the narration of Imam Shafi’i longing to drink Imam Ahmad’s sweat (quoted by ibn ‘Alawi):
وَلَمْ يَكُنْ صَاحِبَ رِحلَةٍ، فَأَمَّا مَا يُرْوَى أَنَّ الشَّافِعِيَّ بعثَهُ إِلَى بَغْدَادَ بِكِتَابِهِ إِلَى أَحْمَدَ بنِ حَنْبَلٍ فغيرُ صَحِيْحٍ
“…the narration which states Imam Shafi’i gave a letter to him for Imam Ahmad and sent him to Baghdad is not authentic.” [Siyar A’lam al-Nubala` 12/587-588]
Furthermore, Imam Ahmad detested such khurafi practices according to those who actually met him:
“Ali al-Tayalisi who is a companion of Ahmad [ibn Hanbal] said: “I wiped my hand over Ahmad bin Hanbal, then I wiped my hand over my body, then he (i.e. Imam Ahmad) got extremely angry and started to shake (i.e. out of anger) and said: “From whom did you get this (i.e. practice) from?” – he strongly condemned it. [al-Qadhi in his Tabaqat al-Hanabilah and al-Buhooti in his Kashhaf al-Qina’]
Note: There are two versions of this story, one claiming that Imam Shafi’i drank Imam Ahmad’s sweat, seeking tabarruk and one where Imam Ahmad is fond of Imam Shafi’is sweat. Most Sufis prefer the one that says that Imam Shafi’i slurped the sweat water of Imam Ahmad. All of that nonsense is fabricated and rejected but this doesn’t prevent even some so-called ‘Atharis’ who are out ‘educating the masses’ by being ‘academic and transparent’ and providing ‘nuances’ for their ‘viewaaas’ with their ana-mafi-maloom Arabic skills:
Oh yeah, before I forget, here is a break-down of nine tabarruk shubuhat used by the Sufi grave kisser Bro Hajji: