Grading: This is a "sahih" hadith, and the chain is "jayyid" (good) because of Safwan bin 'Isa and Ibn 'Ajlan.
Corroborating chains and supporting reports: Another sahih route from Abu Hurayrah will also come later.
Technical note: Abu Salih is Dhakwan al-Samman.

Reference / Source: Imam Ahmad (16/10739), al-Tirmidhi (3557) and al-Nasa'i (1196) narrated it. Imam al-Tirmidhi called it "hasan gharib".

Details of the narration: This narration will come next through Abu Mu'awiyah al-Darir from al-A'mash, in which Sa'd bin Abi Waqqas (may Allah be pleased with him) is made the person of the story.

Variation in the chain: But Hafs bin Ghiyath contradicted Abu Mu'awiyah (in Imam Ahmad 15/9439) and narrated it from al-A'mash, from Abu Salih, from Abu Hurayrah, that the Prophet (peace and blessings be upon him) passed by Sa'd (i.e. the Companion narrating it is Abu Hurayrah).

Important point: Such a disagreement does not harm the authenticity of the hadith, because the chain in any case reaches a Companion, and all the Companions are upright ('udul).
Technical note: Imam al-Daraqutni indicated that Abu Salih may have heard it from more than one Companion, which is why sometimes one name comes and sometimes another.

Reference / Source: Ibn Hibban (884) also narrated it with a "sahih" chain through Muhammad bin Sirin from Abu Hurayrah.

Note / Clarification: "Ahhid, ahhid" means: affirm Allah's oneness (i.e. point with one finger), because the One you are calling upon is One, namely Allah, the Mighty and Majestic.