Ja`far al-Sadiq says `Ali (ra) sent SALAT upon `Umar (ra)

Here’s an authentic narration:

أَنَا أَحْمَدُ بْنُ مُحَمَّدٍ الْعَتِيقِيُّ ، نَا عَلِيُّ بْنُ عُمَرَ الْحَافِظُ ، نَا أَبُو حَامِدٍ مُحَمَّدُ بْنُ هَارُونَ الْحَضْرَمِيُّ ، نَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ الدَّوْرَقِيُّ ، نَا سُفْيَانُ بْنُ عُيَيْنَةَ ، عَنْ جَعْفَرِ بْنِ مُحَمَّدٍ ، عَنْ أَبِيهِ ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ ، قَالَ : قَالَ عَلِيٌّ لِعُمَرَ بْنَ الْخَطَّابِ ، وَهُوَ مُسَجًّى : ” صَلَّى اللَّهُ عَلَيْكَ وَدَعَا لَهُ ” ، قَالَ سُفْيَانُ : قِيلَ لِجَعْفَرِ بْنِ مُحَمَّدٍ : أَلَيْسَ قِيلَ لا يُصَلَّى عَلَى أَحَدٍ إِلا عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ؟ ، قَالَ : هَكَذَا سَمِعْتُ “

[Ahmad bin Muhammad abu Ja`far al-`Atiqi said: abu al-Hasan al-Daraqutni `Ali bin `Umar al-Hafiz al-Baghdadi told us, abu Hamid Muhammad bin Haroun al-Hadrami told us, Ya`qoub bin Ibrahim al-Dawraqi told us, Suffiyan ibn `Uyaynah told us, from Ja`far bin Muhammad, from his father Muhammad bin `Ali, from Jabir ibn `Abdullah al-Ansari that he said: `Ali told `Umar bin al-Khattab while he was on his death-bed: “Salla-Allahu `Alayka, and he made Du`a for him.” Suffiyan said: They said to Ja`far: “Isn’t it said that one cannot send Salat except on the Prophet (SAWS)?” he replied: “This is how I heard it.”]
source: al-Jami` li-Akhlaq al-Rawi by al-Khateeb #1327.
grading: Sahih.

As for what they said regarding not sending Salat upon anyone except the Prophet (SAWS) then it is also mentioned here:

حدثنا أحمد بن معاوية قال, حدثنا سفيان بن عيينة, عن جعفر, عن أبيه, عن جابر بن عبدالله رضي الله عنه: أن عليا رضي الله عنه رأى عمر رضي الله عنه و هو مسجى فقال: صلى الله عليك, ما من الناس … فقال له الحسن بن علي رضي الله عنهما … فقال لا تصل على أحد إلا النبي صل الله عليه و سلم فسكت

In a similar narration of Ahmad bin Mu`awiyah from Suffiyan, it has this additional part at the end:

[… so al-Hasan bin `Ali (ra) told him(His father `Ali) “Do not send Salat upon anyone but the Prophet (SAWS)” so he was silent.]
source: Tareekh al-Madinah by ibn Shubah.

So this same Salat you send upon the Prophet (SAWS) in your prayer, `Ali (ra) sent it upon `Umar (ra).
As for the permissibly of sending Salat upon other than the Prophet (SAWS), they differed:

-Ibn `Abbas, `Umar bin `Abdul-`Aziz, Suffiyan ibn `Uyaynah, abu Hanifa and others said “NO”.

-While others such as al-Hasan al-Basri, Mujahid, Ahmad ibn Hanbal, Ishaq bin Rahaweih ect… said “YES”.

We lean towards the second, as there’s narrations to back it up.