Is the Hadith Authentic?
Question

Does this Hadith apply only to those who visit the sick Muslims to their home?
The Healthcare Professionals especially the doctors & the nurses treat the patients in the hospitals. However, they do this as a part of their profession; not with the intention of visiting sick Muslims.
How can they implement this Hadith & get rewards?
Praise be to Allah.Firstly:
Abu Dawud (3099) and Imam Ahmad in "Al-Musnad" (2/47-48), among others, narrated from `Ali, who said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When a man visits his Muslim brother, he walks in the harvest of Paradise until he sits down, and when he sits down, mercy envelops him. If it is in the morning, seventy thousand angels will pray for him until the evening, and if it is in the evening, seventy thousand angels will pray for him until the morning."
Abu Dawud said after Hadith number (3100): "This was claimed to be narrated from `Ali, from the Prophet (peace and blessings be upon him) through unverified chains." End quote.
This Hadith has been narrated with differences among its narrators; some have transmitted it as a statement of the Prophet (peace and blessings be upon him), and others as a statement of `Ali (may Allah be pleased with him).
At-Tirmidhi said after Hadith (969): "This Hadith has also been narrated from `Ali through other chains, some of which are mawquf (stopped at the Companion) and not raised (to the Prophet)." End quote.
The researchers of Al-Musnad said: "Authentic as mawquf, its narrators are trustworthy, narrators of both Al-Bukhari and Muslim, but there is a difference of opinion on whether it is mawquf or marfu` (attributed to the Prophet), and the mawquf is more authentic." End quote.
And if the mawquf is considered more authentic, then it has the ruling of being marfu` (attributed to the Prophet), because such statements are not made based on opinion; they are from the knowledge of the unseen.
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said:
"An example of a statement that is considered marfu` in ruling, though not explicitly, is when a Companion, who does not take from the Isra'iliyyat (Judeo-Christian traditions), speaks about matters that do not allow for ijtihad (independent reasoning), and are not related to explaining language or clarifying something obscure, such as narrating past events: from the beginning of creation, stories of the prophets, or future events: like battles, tribulations, and the conditions of the Day of Judgment, as well as narrating about specific rewards or punishments for certain actions. It is considered as marfu` because his narration of such matters implies that he has an informant, and matters that do not allow for ijtihad require someone to have established the statement, and there is no one to establish such statements for the Companions except the Prophet (peace and blessings be upon him), or some who narrate from ancient scriptures. Therefore, this precaution is taken to avoid the second category.
If that is the case, then it has the ruling of what would be if he said: The Messenger of Allah (peace and blessings be upon him) said, so it is marfu`, whether he heard it directly from him or through an intermediary." End quote. "Nuzhat An-Nazar" (pp. 132-134).
Secondly:
This Hadith encompasses everyone who intends to visit and care for the sick, seeking reward, whether the visitor is a stranger, a relative, a doctor, or a nurse; because actions are judged by intentions, and everyone will have what they intended; as narrated by `Umar ibn Al-Khattab (may Allah be pleased with him) who said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Actions are (judged) by intentions, and everyone will have what they intended. Therefore, whoever's migration is for worldly gain or to marry a woman, then his migration will be for what he migrated for." Narrated by Al-Bukhari (1) and Muslim (1907).
A single action can sometimes be performed with the intention of two acts of worship or one act of worship and a customary act, and this is a mercy from Allah to this nation.
Shaykh `Abd Ar-Rahman As-Sa`di (may Allah have mercy on him) said:
"From the grace and facilitation of Allah is that a single action can stand in place of multiple actions. So, if one enters the mosque at the time of the prescribed prayer and prays two rak`ahs, intending by them both the prescribed prayer and the greeting of the mosque, he will attain the reward for both." End quote. "Al-Qawa`id wal-Usul Al-Jami`ah" (p. 168).
An example of this is what was reported in the Hadith of Zaynab, the wife of `Abdullah ibn Mas`ud (may Allah be pleased with them both), when she asked the Prophet (peace and blessings be upon him) while desiring to give charity from her wealth: "Would it suffice for me to spend on my husband and the orphans under my care?
He said: Yes, and she will have two rewards: the reward of kinship and the reward of charity." Narrated by Al-Bukhari (1466) and Muslim (1000).
From this principle, a doctor or nurse can rightfully perform their required duties with the intention of visiting and helping the sick.
Shaykh `Umar Sulayman Al-Ashqar (may Allah have mercy on him) said:
"Those who work in worldly professions among Muslims: doctors, engineers, and researchers, can make their work acts of worship by having a righteous intention when performing these tasks, and this does not oblige them to not seek their worldly benefits from these works." End quote. "Maqasid Al-Mukallafin" (p. 400).
Such an intention is among the righteous deeds, leading the doctor and nurse to show kindness to the patients, be patient with them, and not to be annoyed or mistreat them.
And Allah knows best.