The hadeeth about the eclipse of the sun and moon when the Mahdi emerges is not saheeh
Question

                I used to belong to the Ahmadi sect but praise be to Allaah, Allaah has guided me to the Sunnah, myself, my mother and two of my brothers. I ask Allaah to guide my father and all my brothers and sisters.

 I have read a hadeeth which says that one of the signs of the Mahdi will be that the sun and moon will be eclipsed in the month of Ramadaan, when he first appears. Is this hadeeth saheeh (sound) or da’eef (weak)?.
Praise be to Allah.We praise Allaah for having guided you and 
    saved you from the danger of this deviant sect, and we ask Him to bless your father and all your family with right guidance. 
With regard to the report that you refer to, 
    this is not a hadeeth from the Prophet  (peace and blessings of Allaah be upon him), rather it was narrated by al-Daaraqutni in his Sunan 
    (2/65) from the words of Muhammad ibn al-Haneefiyyah (the son of ‘Ali ibn Abi Taalib). Al-Daaraqutni said: Abu Sa’eed al-Astakhri told us, 
    Muhammad ibn ‘Abd-Allaah ibn Nawfal told us, ‘Ubayd ibn Ya’eesh told us, Yoonus ibn Bakeer told us, from ‘Amr ibn Shamr, from Jaabir, from 
    Muhammad ibn ‘Ali who said: 
“Our Mahdi will have two signs, which have 
    never appeared since the heavens and earth were created. The moon will be eclipsed on the first night of Ramadaan and the sun will be eclipsed 
    halfway through, and this has never happened since Allaah created the heavens and the earth.” 
This report is mawdoo’ (fabricated) and is 
    falsely attributed to Muhammad ibn ‘Ali (ibn al-Haneefiyyah), may Allaah have mercy on him. 
Dr. ‘Abd al-‘Aleem ‘Abd al-‘Azeem al-Bastawi 
    said in his book al-Mawsoo’ah fi Ahaadeeth al-Mahdi al-Da’eefah wa’l-Mawdoo’ah, p. 169: 
“Its isnaad includes Yoonus ibn Bakeer ibn 
    Waasil al-Shaybaani, Abu Bakr al-Jammaal al-Kufi (d. 199 AH), who makes mistakes; and ‘Amr ibn Shamr al-Ja’fi al-Kufi, the Shi’i Abu ‘Abd-Allaah, 
    who is a fabricator of reports. 
Al-Sulaymaani said: ‘Amr used to fabricate 
    ahaadeeth to support the Raafidis (Shi’is).  
Al-Jawzjaani said: He is a liar and deviant. 
Al-Haakim said: There were many fabricated 
    reports from Jaabir al-Ja’fi, and no one narrated these false and fabricated reports from Jaabir except him. 
Ibn Hibbaan said: He is a Raafidi (Shi’i) who 
    used to defame the Sahaabah and narrate fabricated reports from trustworthy narrators. 
Abu Haatim said: His hadeeth is munkar and 
    da’eef, and should be ignored; they [the scholars] ignored him. 
He was described in such negative terms by 
    more than one of the scholars, including al-Bukhaari, al-Nasaa’i, Ibn Sa’d, al-Daaraqutni and others. 
Jaabir is al-Ja’fi, who is matrook 
    al-hadeeth. Shu’bah, Wakee’ and al-Thawri described him as trustworthy, but Ibn Ma’een, Abu Haneefah, Layth ibn Abi Sulaym, al-Jawzjaani, Ibn 
    ‘Uyaynah, Ibn Kharraash, Sa’eed ibn Jubayr and others described him as a liar, and many described him as da’eef (weak). 
Al-Dhahabi said: Only Shu’bah described him 
    as trustworthy so he was at odds with other scholars, and the scholars of hadeeth ignored his narrations. 
Ibn Hajar said: he is da’eef (weak) and a 
    Raafidi (Shi’i). 
In conclusion: this report is mawdoo’ 
    (fabricated) and the problem in its isnaad is ‘Amr al-Ja’fi. 
Al-‘Azeemabaadi said: ‘Amr ibn Shamr from 
    Jaabir: both of them are da’eef (weak) and cannot be quoted as evidence.” 
And Allaah knows best.