Who is "AHMAD" in Surah As-Saff (61:6)?!

Allah s.w.t said:

"وَإِذْ قَالَ عِيسَى ابْنُ مَرْيَمَ يَا بَنِي إِسْرَائِيلَ إِنِّي رَسُولُ اللَّهِ إِلَيْكُمْ مُصَدِّقًا لِمَا بَيْنَ يَدَيَّ مِنَ التَّوْرَاةِ وَمُبَشِّرًا بِرَسُولٍ يَأْتِي مِنْ بَعْدِي اسْمُهُ أَحْمَدُ ۖ فَلَمَّا جَاءَهُمْ بِالْبَيِّنَاتِ قَالُوا هَٰذَا سِحْرٌ مُبِينٌ" الصف 6

"And remember, Jesus, the son of Mary, said: "O Children of Israel! I am the messenger of Allah (sent) to you, confirming the Law (which came) before me, and giving Glad Tidings of a Messenger to come after me, whose name shall be Ahmad." But when he came to them with Clear Signs, they said, "this is evident sorcery" As-Saff (61:6)

When Sahaba migrated to Habasha, there was an interview between Jafar Ibn Abi Taleb r.a and Najashi (king of Habasha). This is a part of it:

قالَ لَهُ النَّجاشيُّ : وما ذاكَ ؟ قالَ : إنَّ اللَّهَ بعثَ فينا رسولَهُ ، وَهوَ الرسولُ الَّذي بشرَ بهِ عيسَى ابنُ مريمَ برسولٍ يأتي من بعدي اسمُهُ أحمدُ

Translation: 

Najashi said: "and what's this?", he (Jafar Ibn Abi Taleb) said: "Allah has sent his messenger to us, and He (s.a.w) is the messenger whom Issa Ibn Mariam has promised to come after him, His name is Ahmad."

التخريج : أخرجه الحاكم في ((المستدرك)) (2/ 338)، وابن أبي شيبة (14/ 346) واللفظ له، وأبو نعيم في ((حلية الأولياء)) (1/ 114). باختلاف يسير

References: Almostadrak (2/338), Ibn Abi Shai'bah (14/346), Heliat Al Aw'lea (1/114).