Al-Manāwī, in his Sharḥ al-Jāmiʿ al-Ṣaghīr, reached the same verdict, as did others. If you wish to examine the matter further, consult Takhrīj Aḥādīth al-Mushkil, p. 63, and Silsilat al-Aḥādīth al-Ḍaʿīfah, no. 5069.

Under the heading “12. Supplicating for the Zakah-Payer,” he cites: “On the authority of ʿAbd Allāh ibn Abī Awfā: whenever the Messenger of Allah ﷺ was brought a charity, he would say, ‘O Allah, send blessings upon them.’ When my father brought his charity, he said, ‘O Allah, send blessings upon the family of Abū Awfā.’ ” He adds that it was narrated by Aḥmad and others.

I say: The author has strayed far, for this ḥadīth is found in the two Ṣaḥīḥs, as noted in al-Muntaqā and elsewhere. It is impermissible, when a report is in the Ṣaḥīḥayn or in either of them, to attribute it to other collections except incidentally or for an additional wording, because that suggests its authenticity is not firmly established. The ḥadīth is also recorded in Irwāʾ al-Ghalīl, no. 853, and Ṣaḥīḥ Abī Dāwūd, no. 1415.

He then writes under the heading “Combining the Two Monetary Species”: “If a person possesses less than the niṣāb of gold and likewise less than the niṣāb of silver, one is not added to the other to complete a niṣāb, for they are two different kinds; one is not combined with the other, just as cattle are not combined with sheep….”

I say: Citing a ḥadīth here is better than relying on analogy. Ibn Ḥazm—after refuting those who allow the combination and demolishing their view by rational argument (6/83)—said:

“Our proof that it is unlawful to aggregate them for zakah is the statement of the Messenger of Allah ﷺ: ‘There is no charity due on less than five awqiyahs of silver.’ Whoever combines gold and silver has thereby made zakah obligatory on less than five awqiyahs, which is a plain contradiction of the command of the Messenger of Allah ﷺ and a legislation to which Allah the Exalted has given no leave.

“They even authenticate the report that cancels zakah on less than twenty dinars, yet they impose it on less than that—an astonishing inconsistency indeed!”