Grading: This narration is "sahih" as "mawquf" (his own statement) on 'Abdullah bin 'Amr bin al-'As (may Allah be pleased with him), except for the words: "Whoever crosses the sea, it is as though he has crossed all the valleys".
Technical note / Defect ('illah): The present chain is "da'if" (weak) because the narrator 'Abdullah bin Salih (the scribe of al-Layth) had a poor memory. He erred in the chain of this hadith in two places: first, he dropped the intermediary between Yahya bin Sa'id al-Ansari and 'Ata' bin Yasar; second, he made it "marfu'" (a statement of the Prophet (peace and blessings be upon him)). In contrast, Sufyan al-Thawri narrated it from Yahya bin Sa'id al-Ansari, from an unnamed person, from 'Ata' bin Yasar, as "mawquf" on 'Abdullah bin 'Amr. However, this report is narrated through another sahih chain from 'Ata' bin Yasar from 'Abdullah bin 'Amr as "mawquf", so its being "mawquf" is what is correct.
Note / Clarification: This investigation makes clear that al-Mundhiri's agreement in "al-Targhib" (2/199) with al-Hakim's authentication, and his statement that "the criticism of 'Abdullah bin Salih does no harm, because al-Bukhari relied on him as proof", is not a view that can be accepted.

Reference / Source: It was recorded by Ibn Hibban in "al-Majruhin" (2/42), al-Tabarani in "al-Mu'jam al-Awsat" (3144) and "al-Mu'jam al-Kabir" (14581), Ibn Shahin in "al-Targhib fi Fada'il al-A'mal" (436), Abu al-Husayn bin Bishran in "al-Fawa'id" (89), Abu al-Qasim bin Bishran in "al-Amali" (1530), and al-Bayhaqi in "al-Sunan al-Kubra" (4/334) and "Shu'ab al-Iman" (3917), via 'Abdullah bin Salih with this same chain.
Corroborating chains and supporting reports: 'Abd al-Razzaq (9630) and Ibn Abi Shaybah (5/315) narrated it via Sufyan al-Thawri, in which Yahya bin Sa'id says, "An informant told me" (an unnamed intermediary), from 'Ata' bin Yasar, from 'Abdullah bin 'Amr, as "mawquf".

Grading: This chain of it is "sahih".
Reference / Source: Sa'id bin Mansur narrated it in his "Sunan" (2395) through Ya'qub bin 'Abd al-Rahman and 'Abd al-'Aziz bin Abi Hazim, from Abu Hazim, from 'Ata' bin Yasar, from 'Abdullah bin 'Amr, as "mawquf"; however, the words about "crossing the sea and the valleys" are not in it.

Grading: Its chain is "hasan".
Important point: The mention of the "ma'id" (one who becomes dizzy) at sea is established in a marfu' hadith of Umm Haram bint Milhan with Abu Dawud (2493) in these words: "The one who becomes dizzy at sea and is struck by vomiting has the reward of a martyr".

Note / Clarification: "al-Ma'id" is an active participle derived from "mada yamidu"; it means a person whose head starts spinning because of stomach upset and nausea caused by the sea wind.

Note / Clarification: The word "ajaza" means "to cross" or "to traverse".

Note / Clarification: "al-Mutashahhit" means a person who is soaked in blood.