That Ali, may Allah be pleased with him, said: The Sunnah is to place one hand over the other in prayer below the navel. (1)

757 — Muhammad ibn Qudamah — meaning ibn A'yun — narrated to us, from Abu Badr, from Abu Talut 'Abd al-Salam, from Ibn Jarir al-Dabbi, from his father, who said:

I saw Ali, may Allah be pleased with him, holding his left hand with his right hand upon the wrist, above the navel. (2)

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(1) Its chain of transmission is weak: 'Abd al-Rahman ibn Ishaq is weak, and Ziyad ibn Zayd al-Suwa'i is unknown.
It was also recorded by Ibn Abi Shaybah 1/391, 'Abdullah ibn Ahmad in his additions to "al-Musnad" (875), al-Daraqutni (1102), al-Bayhaqi 2/31, and al-Diya' in "al-Mukhtarah" (771) (and he erred in including it in "al-Mukhtarah"), and (772) via two paths from 'Abd al-Rahman ibn Ishaq, with this chain.
It was also recorded by al-Daraqutni (1103) and al-Bayhaqi 2/31 via the path of Hafs ibn Ghiyath, from 'Abd al-Rahman ibn Ishaq, from al-Nu'man ibn Sa'd, from Ali. Al-Nu'man ibn Sa'd is also unknown.

(2) Its chain of transmission is weak: Jarir al-Dabbi and his son — whose name is Ghazwan — are both unknown, and they have nothing in the six canonical books except this hadith. Abu Badr is Shuja' ibn al-Walid, and Abu Talut is 'Abd al-Salam ibn Abi Hazim.
It was also recorded by Ibn Abi Shaybah 1/390 from Waki', and al-Bayhaqi 2/29-30 via the path of Muslim ibn Ibrahim, both from 'Abd al-Salam ibn Abi Hazim, with this chain, with the wording: "When he stood for prayer and said the takbir, he struck his right hand upon his left wrist, and he would remain like that unless he adjusted his garment or scratched his body." Neither of them mentioned "above the navel" in it, and al-Bayhaqi said: Its chain is good!
With this wording — that is, without his statement "above the navel" — al-Bukhari suspended it in his "Sahih" at the beginning of the Book of Actions in Prayer. And al-Hafiz Ibn Hajar connected it in "Taghliq al-Ta'liq" 2/442 via the path of Muslim ibn Ibrahim, and deemed its chain good!