Proof of the existence of Musnad Zayd before 300 AH?

Salam Alaykum! With respect to the Musnad of al-Imām Zayd, peace be upon him, I was wondering if there is any early reference to it? I am yet to see any of our early Imāms mention it, or really anyone prior to al-Imām ʿAbdullāh b. Ḥamza speak of it as an existing work. Could you kindly point me towards any evidence that indicates there was a thing called "Musnad al-Imām Zayd" that was referenced around the years 300 AH and prior? If not, then I believe this Musnad is just a compilation of the ḥadīths we have in say al-Aḥkām, and the Amālī works compiled into one book and with the isnāds shortened. 

wa alaykum as salaam wa rahma!

Thank you for your question! I would say that I have not found any early evidence of the Musnad in other texts. Even in the Ahkaam, Imam al-Haadi (as) does not reference a compilation of the reports of Imam Zayd (as). Rather, he relates reports on the authority of Imam Zayd (as), some of which exists in the Musnad, some of which doesn't and some of which contradicts what's in the Musnad. I touch upon this in the introduction of our translation. 

This does raise questions as to the compilation of the Musnad before Imam al-Haadi (as). It is generally thought that the text circulated around Kufa, Iraq. However, there doesn't seem to have been a presence of such work in the Hijaz or outside of Kufa. I believe that the book was said to have reached Zaydi Yemen for the 1st time during the imamate of Sayyid Imam al-Mutawakkil Ahmed b. Suleiman (as). Prior to then, I haven't seen or heard of the book being referenced by our imams as such. 

Hopefully, this helps! And Allah knows best!

IRS 

wa alaykum as salaam wa rahma!

Thanks for the reference, yaa ustaadh! However, I wouldn't see this as a reference to the Musnad Zayd per se. Keep in mind that Imam al-Haadi, upon him be peace, also related from Abu Khaalid in his Ahkaam, but the reference was not from Imam Zayd, upon him be peace. Abu Khaalid is seen as a reliable source of narrations according to our imams. However, his narrations cannot be seen to be exclusive to the Musnad Zayd. 

And Allah knows best!

IRS